1. Field of the Invention
This invention relates to a foam and a method of manufacturing the foam.
2. Description of the Related Art
In recent years, from the viewpoint of environmental protection, there is an increasing demand for biodegradable resins and molded products thereof which can be decomposed in the natural environment, so that studies on biodegradable resins such as aliphatic polyester, etc., are now actively performed. Although resins of this kind are incapable of being decomposed under ordinary environments of use, the resins can be decomposed by the action of microorganisms into water and carbon dioxide under composting conditions of high temperature and high humidity. Therefore, biodegradable resins are expected to be a promising material for solving the problem of pollution since they are low in environmental load in the disposal thereof.
Most of biodegradable resins are synthesized from petroleum. When the exhaustion of petroleum as well as the problem of global warming caused by an increase in quantity of carbon dioxide throughout the world due to the disposal of used resins are taken into account, the idea to biodegrade materials that have been synthesized from petroleum is undesirable in viewpoint of carbon neutrality.
Under the circumstances mentioned above, it is now desired to develop biodegradable resins and molded products thereof, which can be synthesized and produced by using materials of natural origin without necessitating employment of fossil resources such as petroleum resources.
On the other hand, foam which is molded by using a resinous material is generally employed as a packaging material or as a heat insulating material by taking advantage of its shock absorbing and heat insulating properties. Even in the case of foam, it is now considered desirable to employ materials of natural origin without necessitating the consumption of petroleum resources. Incidentally, even in the case of foam made of natural origin, since most of the foaming agents currently employed for foam are made of chemical foaming agents, they are undesirable because of the problem of global warming and pollution due to chemical materials constituting the foaming agents.
As for biodegradable resins which are recently employed, there is known a foamed body of starch. Since this foam is formed of an organic material or a plant as a raw material, this foam can be decomposed by burying it in soil after use. Further, this foam is soluble in water, it can be disposed by dissolving it in water after use. However, there are problems that the water-solubility rate of this foam made from starch is not so fast as expected and that this foam cannot be dissolved in water unless the water is hot.
Under the circumstances, there has been developed a technique for manufacturing a foam by using alginic acid which is a water-soluble polymer. The alginic acids to be employed in this technique are included, in large quantities, mainly in seaweeds. Since there is no means to effectively utilize the seaweeds containing the alginic acids, the seaweeds are discarded without being utilized. Since the seaweeds containing the alginic acids are available in large quantities, the seaweeds are deemed to be preferable for use as a raw material in the manufacture of foam even when environmental problems are taken into account.
Foam of alginic acid according to the prior art has been manufactured through gelation of a water-soluble polymer. Because of the large quantity of water included in this water-soluble polymer, this water-soluble polymer is dried at a temperature of about 20 to 60° C. instead of heating it at an elevated temperature which is higher than the solidification temperature of protein. As a result, it takes a long time for the volatilization of water, thus degraded the working efficiency in the manufacture of foam. Although it is possible to employ protein as a polymer for constituting a foam, the cells to be formed by protein are more likely to be defoamed. Moreover, the number of cells gradually decreases with time, thereby making it impossible to retain the configuration of foam for a long period.